packages: 'patch-and-repack' compresses tarballs again.

* guix/packages.scm (patch-and-repack): Add missing "a" in "tar cvfa".
Fixes a regression introduced in
3e95125e9b whereby we'd always create
uncompressed tarballs.

guix/packages.scm

@@ -628,7 +628,7 @@ specifies modules in scope when evaluating SNIPPET."
                                           #:fail-on-error? #t)))))
               (apply invoke
                      (string-append #+tar "/bin/tar")
-                     "cvf" #$output
+                     "cvfa" #$output
                      ;; avoid non-determinism in the archive
                      "--mtime=@0"
                      "--owner=root:0"